A long-overdue renovation at the New York Studio School has turned up buried treasure: a mural painted by Cecily Brown when she was a student there in the 1990s, hidden behind layers of plasterboard for decades. Builders found it within a day of uncovering two further murals by Robert E. Locher, commissioned a century ago by the school's founder, Gertrude Vanderbilt Whitney.

The five buildings at 8 West 8th Street in Greenwich Village are among the most storied addresses in American art. Whitney set up her sculpture studio in a converted stable and hayloft on MacDougal Alley in 1907, then bought up four neighbouring brownstones one by one. She opened the Whitney Museum of American Art there in 1931; after the museum moved uptown, the fledgling New York Studio School took over the compound in 1967. Edward Hopper sketched, John Cage lectured and Philip Guston taught within its walls, and the whole site is now a National Historic Landmark.

The school's director, the artist Matthew Ritchie, made renovation his priority after taking charge last year, when the roof and studio skylights leaked, century-old wiring had deteriorated and wheelchair access was extremely limited. Working with Matthew Baird of Baird Architects on a $2.2 million overhaul, the first phase, due to finish this autumn, doubles the public exhibition space and adds an accessible lecture and performance hall.

Brown had told Ritchie she painted her first mural on a school wall as a student but could not remember where. The rediscovered work, showing disembodied penises, two of them appearing to kiss, now sits in the expanded main-floor gallery, discreetly screened by a shallow cabinet. The Locher murals, one glimmering green and the other a trompe l'oeil of Doric columns, were found where a bathroom and hallway are planned, and the layout was adjusted to keep them in place. Less happily, the beams beneath Robert Winthrop Chanler's flame-motif fireplace were found to be cracking and needed emergency shoring.